Problem 38 - Objective Biology for NEET

Question

From which part of nephron ADH stimulates reabsorption of water and electrolyte? (a) PCT (b) \(\mathrm{HL}\) (c) Distal tubules (d) Bowman capsule

Step-by-Step Solution

Answer

The answer is (c) Distal tubules.

Step 1: Understanding the Role of ADH

ADH or antidiuretic hormone plays a crucial role in helping kidneys manage the amount of water in your body. The main way it does this is by regulating water reabsorption at the Distal Convoluted Tubule(DCT) and Collecting Duct(CD).

Step 2: Determine the Function of Each Nephron Part

Each part of the Nephron plays different roles. PCT is where most of the reabsorption takes place, HL is involved in establishing the medullary osmotic gradient which helps in the concentration of urine whereas Bowman capsule is associated with filtrate formation.

Step 3: Identify the Correct Answer

Comparing ADH's role with nephron parts functionality, it only stimulates reabsorption of water and electrolyte at the distal tubules, by making the cell membrane more permeable to water.

Key Concepts

Nephron

At the heart of the kidney's filtration system lies the nephron, a microscopic structure that plays a pivotal role in purifying the blood and maintaining homeostasis. These tiny units, numbering in the millions in each kidney, are made up of various segments, each tasked with specific functions crucial for urine formation and overall electrolyte balance.
The nephron begins with the Bowman's capsule, which encloses a network of tiny blood vessels called the glomerulus. Together, they form the filtration barrier for the primary processing of blood filtrate.

• The Proximal Convoluted Tubule (PCT) comes next, responsible for reabsorbing nutrients, salts, and water from the filtrate, returning them into the bloodstream.
• Following the PCT is the Henle's Loop (HL), playing a key part in establishing the kidney's ability to produce urinary concentration gradients.
• The distal sections, including the Distal Convoluted Tubule (DCT) and the Collecting Duct (CD), are integral in fine-tuning the reabsorption of water and electrolytes.

The orchestration of these segments ensures that waste is adequately removed, while essential substances are retained, maintaining balance within the body.

Water Reabsorption

Water reabsorption in the nephron is a finely tuned process that allows the body to conserve water as needed. Controlled by several hormones, including ADH, the mechanism targets specific nephron parts, primarily the distal tubules and collecting ducts.
ADH, or antidiuretic hormone, is pivotal for water reabsorption. It acts on the cells of the distal tubules and collecting ducts, increasing their permeability to water. This action is vital, especially in situations where the body needs to conserve water, such as dehydration or high plasma osmolality.

• ADH impacts the cells' membrane by inserting water channels known as aquaporins, facilitating water flow back into the bloodstream and reducing urine volume.
• This process ensures water conservation, which is crucial for maintaining blood pressure and hydration levels.

Through this hormonal regulation, the body efficiently adjusts water reabsorption based on immediate physiological demands, effectively managing internal water balance.

Electrolyte Balance

Electrolyte balance within the body is critical for several physiological functions, including nerve transmission, muscle contraction, and maintaining acid-base balance. The nephron, specifically its distal segments, plays a key part in managing the levels of electrolytes such as sodium, potassium, and calcium.
The distally located segments of the nephron, namely the distal convoluted tubule and the collecting duct, actively regulate the concentration of these electrolytes under hormonal influence. ADH facilitates water reabsorption, indirectly impacting electrolyte concentration by controlling water flow:

• Through ion exchange processes in the distal tubules, sodium retention is often coupled with the excretion of potassium.
• Hormones like aldosterone work alongside ADH to modulate this exchange, promoting sodium reabsorption and potassium excretion.

These processes are essential for fluid balance and are tightly regulated to adapt to the body's changing needs, ensuring stability in electrolyte concentrations and, consequently, a well-functioning internal environment.